Green tatties

Before going on the holidays I dug up some volunteer tatties,washed them and left on colander on the worktop.I came back from the holidays and they turned green.
The same with bought tatties-turned green.

Potatoes turn green when exposed to the light, this is the purpose of earthing up, to prevent light getting to tubers near the surface. It's true that the green bits are poisonous, and even once you take the green bits off, the rest of the tuber contains traces of the poisonous stuff. As potatoes are related to deadly nightshade, it's best not to take too many risks

Really not to be recommended, C:
"Potato poisoning is rare, but it does happen from time to time. Death normally comes after a period of weakness and confusion, followed by a coma. The majority of cases of death by potato in the last fifty years in the USA have been the result of eating green potatoes or drinking potato leaf tea."Top 10 Poisonous Foods We Love To Eat - Listverse
b.

Potatoes turn green if left exposed to light Coreopsis. They are poisonous and taste horrible. You may have escaped poisoning on this occasion but they are very highly implicated in foetal defects. Hope no one is pregnant. Please don't eat green potatoes again.

...All potatoes contain substances known as glycoalkaloids. The most common in potatoes are solanine and chaconine. Light reacts with these substances to turn potato flesh green. The green parts of a potato are a guide to where these substances tend to accumulate which is just below the potato skin. Ingested in large quantities these toxic compounds could lead to coma or even death. In smaller quantities the most likely effects are diarrhoea, headaches and stomach cramps. Fortunately these toxic substances are eliminated by thorough cooking. Solanine is the toxic substance found in deadly nightshade which indicates the potential seriousness of eating uncooked potatoes which are, in any case, hard to digest and unpleasantly flavoured. The only recent cases of solanine poisoning lined to potatoes have been the result of drinking potato leaf tea or green potatoes. When potatoes turn green it is an indication of higher concentrations of solanine...

Raw potatoes aren't good for you either, green or otherwise.

All potatoes that you intend to eat should be kept in the dark to prevent them from turning green, and any green ones should be discarded and (obviously) should definitely not be fed to children!